Shrinkage compensator for building tiedowns

ABSTRACT

An automatic, self-energized, length extending, take-up unit for general use in wall hold down systems. The take-up unit is generally installed as a separate element, independent of threaded retaining elements of a hold down system. One embodiment includes an internal, preloaded, coil spring to rotate a threaded sliding member relative to a threaded base member, thereby to extend a height of the take-up unit. A trigger mechanism may be included to prevent unintended actuation of the unit. Additionally, a safety mechanism may be included to prevent inadvertent separation of a base member and a sliding member, thereby to prevent loss of a preload in an energizing spring.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/146,531 filed on May 15, 2002 and now U.S. Pat. No. 6,585,469entitled SHRINKAGE COMPENSATOR FOR BUILDING TIEDOWNS.

BACKGROUND

1. The Field of the Invention

This invention pertains to building construction, and, moreparticularly, to novel methods and apparatus for anchoring buildingwalls to foundations and lower floors thereof. The invention provides anautomatic adjusting mechanism to remove slack in a hold down systemcaused by wood shrinkage over time or wood crushing caused byearthquakes.

2. The Background Art

Wood products change dimensions as moisture content changes. Floorsystems using solid sawn joists typically shrink approximately fivepercent in dimensions across the grain. Under certain conditions theyhave been known to shrink six and one-half percent within a year. Thisshrinkage is typically part of the overall process and condition called“settling.” Settling actually includes both settling of foundations, aswell as settling of walls due to shrinkage.

Testing and load rating has been completed for shear walls mounted tosolid underlying surfaces. The solid surfaces are typically comprised ofsteel, concrete, or both. In tests wherein a wall is constructed, andimmediately tested thereafter, test results are substantially betterthan those for walls that have existed over time. In a typical practice,a sill plate anchor or lower anchor is a threaded rod or an anchoredstrap capturing the base plate or sill plate of a wall (the bottom,horizontal member above which the studs extend vertically). Over time,ranging from several months to several years, wood loses moisture,shrinks, and the building settles. Threaded rod type anchors becomeloose. Strap type anchors buckle if positively engaged and become loadedin compression, or the like.

Current tiedown systems (including rods, straps, and the like) do notprovide a solution for this problem. After a building “settles” the wallcan lift before it will re-engage the hold down structure before thetiedown is even loaded to begin resisting movement of the wall.Substantial building damage can result before the anchoring hardware isloaded (in tension). Hardware that does not immediately engage the baseof an anchored wall can result in a 50 percent to 70 percent loss inlateral, load-bearing capacity.

The problem arises, typically, in wind storms of great power, or inearthquake conditions. A building under such circumstances may beviolently loaded or shaken back and forth in a lateral direction withrespect to the extent of the wall. If a shearwall is tightly restrainedby its base to a foundation, loads may be smoothly transferred from ahorizontal to a vertical direction. Loads are resolved in thefoundation, where they appear as tension and compression forces.

Buildings are often composed of long walls, (walls with a length greaterthan the height) and short walls (walls that have a length shorter thanthe height). The uplift load on a particular wall is inverselyproportional to the length of the wall. Tall narrow shear walls (ascommonly found in nearly all homes) act as lever arms and tend tomagnify the input load. In certain instances and depending upon wallstructural configuration, the actual load on the anchoring system may bemagnified to several times the original load. Gaps caused by woodshrinkage may further introduce an undesirable shock load to theanchoring system as the gaps are closed and the anchor system is finallyloaded.

However, the as-built building is generally not the building that willbe sustaining loads induced by earthquake shaking or by wind. Woodcomponents of the building structure, including floors, sill plates, topplates, and studs, will shrink. Shrinkage varies greatly but it rangestypically from about one-quarter inch under the best of conditions, towell over one inch.

Moreover, under load, wood crushes or collapses in compression under theloading of a wall. Neither shrinkage nor crushing are well-accommodatedor otherwise resolved in currently available systems. These problemslead to a significant reduction in the lateral, load-bearing capacity ofshearwalls. Typically, based on testing, load-bearing capacityreductions range from about 30 percent to about 70 percent, depending onwhether the rating used corresponds to building codes for propertypreservation, or life safety.

A better hold down or tiedown system including an improved take-up isneeded to accommodate shrinkage of building materials. An improvedtiedown system with such an improved take-up mechanism will improve thestrength of shear walls subject to shrinkage of constituent materials.

BRIEF SUMMARY AND OBJECTS OF THE INVENTION

In accordance with the above-identified needs, it is an objective toprovide a self-powered take-up system for removing slack from betweenshearwalls and their anchors. It is an objective to provide a highreliability with a 30 year minimum life expectancy. It is desired thatthe apparatus be a non-reversing (locking in the extended position)design that may be shipped in a cocked, or preloaded position, ready tofire or deploy to extend in height, and having a trigger mechanism toactuate operation thereof. It is desirable to provide a simple mechanismthat may be activated in the field with a minimum of skill and tools. Incertain embodiments it is desirable to provide smooth and continuousmotion of the take-up process, involving no stepped functions.Nevertheless, in other embodiments, step functions may be desirable. Itis desired to minimize backlash to within thousandths of an inch.

It is desired to provide an apparatus that may be concentrically loaded,and thus able to tolerate and better center eccentric loads. Inpractice, few items are ever installed fully squared, and the take-upapparatus in accordance with the invention is preferably able tofunction at loads equal to the full tiedown design load for any anchorto which the take-up is attached. Ultimately, the tiedown shouldwithstand the full ultimate load of a corresponding tiedown apparatuswithout jamming or deflecting substantially (e.g. more than onesixteenth of an inch).

In certain embodiments, the take-ups may be stackable to provideadditional take-up capacity where more shrinkage, due to more buildup ofwood, occurs. It is desired to provide standard sizes adapted to fitcommonly used anchors and to fit within the space currently availablefor tiedown mechanisms. Thus, the take-up according to the instantinvention may be sized for use with virtually any common size of tiedownsystem, or even for use in specially sized systems.

Consistent with the foregoing objects, and in accordance with theinvention as embodied and broadly described herein, an apparatus andmethod are disclosed, in suitable detail to enable one of ordinary skillin the art to make and use the invention. In certain embodiments anapparatus and method in accordance with the present invention mayinclude an extender of monotonically increasing length (or height)fitted between an anchor and a wooden structure for taking up the spacecaused by wood shrinkage or crushing.

One embodiment of an apparatus according to the present inventionincludes a take-up unit having a hole therethrough to accommodate ananchor bolt of a hold down system. The take-up unit is generallystructured to be interposed in compression between a surface to beretained and a retention nut carried in threaded engagement by theanchor bolt. A key functionality of a take-up unit is the capability ofautomatically increasing in height. Such increase in height may beaccomplished in discrete steps, or as a continuous and smooth function.

A take-up unit typically includes a base member and a sliding member.The base and sliding members are generally cooperatively structured sothat relative sliding motion between the base and sliding members causesa change in height of the take-up unit. The base and sliding membersalso typically have an interface therebetween adapted to retain themembers in close sliding relation. One such interface might be amutually engaged thread interface between the base and the slidingmembers. A second such sliding interface between a base and slidingmember is a dovetail joint.

An exemplary take-up unit also typically includes a biased elementconfigured as an expansion producing element. One practical and highlypreferred expansion element is embodied as a coil spring, the springbeing interposed between the base and sliding members and preloaded tourge the members toward a position of increased unit height. Alsopreferably included is a trigger mechanism operable to maintain the unitin an installation configuration having a first height. The triggermechanism is generally releasable to deploy the unit for an automaticincrease in height to a second height subsequent to installation of theunit in a hold down system.

A take-up unit according to the present invention may also include asafety mechanism which may be provided by an interference betweenstructure carried by the sliding and base members. The safety mechanismmay be engaged, or enabled, during manufacture of the take-up unitsubsequent to preloading one or more springs. One function of a safetymechanism is to prevent unintentional disassembly of the unit. A safetymechanism may determine a maximum range of relative motion between baseand sliding members. An exemplary safety mechanism is formed by a flangecarried by the sliding member, the flange being deformed into positionto interfere with structure carried by the base member subsequent toassembly of the unit. The interference created between the flange andbase member structure prevents unintentional disassembly of the unitthereafter. A snap ring may also be employed to perform the samefunction as the flange.

A sliding member may also include a cover to form a debris-shield overan advancing interface between the members. The advancing interface maybe defined as including at least a portion of that surface whichproduces an additional increase in unit height, the advancing interfacebeing in addition to any surface in contact between the members while inan installation configuration. A take-up unit may be structured andarranged for stacking a plurality of units in-line in a hold down systemto effect an increased range of total extension height.

A take-up unit may be fashioned to have a sliding member that includes aright-hand threaded hole for engagement with a commercial anchor bolt.The sliding member typically includes a bearing surface against which toreceive a jam nut, the jam nut also being in engagement with the anchorbolt. To further prevent inadvertent rotation of the sliding memberabout the anchor bolt under the action of a preloaded spring, a threadhaving a different direction than the anchor bolt thread is preferablyprovided between the base and sliding members. To accommodatecommercially available right hand threaded anchor bolts, a left-handthreaded engagement may preferably be provided as a slidable captureinterface between sliding and base elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and features of the present inventionwill become more fully apparent from the following description andappended claims, taken in conjunction with the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are, therefore, not to be considered limiting of itsscope, the invention will be described with additional specificity anddetail through use of the accompanying drawings in which:

FIG. 1 is a perspective view in elevation of an apparatus providingautomatic take-up in accordance with the present invention, depicted intwo typical deployment arrangements illustrating take-up systems toaccommodate shrinkage;

FIG. 2 is a perspective view from above of an apparatus shown in FIG. 1,in a contracted height configuration and with a safety trigger engaged;

FIG. 3 is a perspective view in elevation of an apparatus of FIG. 1, inan expanded height configuration and with a safety trigger disengaged;

FIG. 4 is an exploded assembly view in perspective of an apparatus ofFIG. 1;

FIG. 5 is a cross-section view of an apparatus shown in FIG. 1,illustrating manufacturing details of one way to provide a positiverestraint against disassembly;

FIG. 6 is a perspective view from below of the apparatus of FIG. 5;

FIG. 7 is a perspective view in elevation illustrating two stackedapparatus of FIG. 1, being configured for increased range of adjustment;

FIG. 8A is a cross-sectional view in elevation of a third take-upmechanism according to the present invention, illustrating a minimuminstallation height and a safety trigger mechanism;

FIG. 8B is a cross-sectional view in elevation of the apparatus of FIG.8A, illustrating a maximum take-up height.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be readily understood that the components of the presentinvention, as generally described and illustrated in the Figures herein,could be arranged and designed in a wide variety of differentconfigurations. Thus, the following more detailed description of theembodiments of the system and method of the present invention, asrepresented in 1 through 8B, is not intended to limit the scope of theinvention. The scope of the invention is as broad as claimed herein. Theillustrations are merely representative of certain, presently preferredembodiments of the invention. Those presently preferred embodiments ofthe invention will be best understood by reference to the drawings,wherein like parts are designated by like numerals throughout.

Those of ordinary skill in the art will, of course, appreciate thatvarious modifications to the details of the Figures may easily be madewithout departing from the essential characteristics of the invention.Thus, the following description of the Figures is intended only by wayof example, and simply illustrates certain presently preferredembodiments consistent with the invention as claimed.

Referring to FIG. 1, a wood shear wall 8 is an engineered assembly oflumber, plywood (or OSB), nails and attachment hardware. Shear wallsresist in-plane wind or seismic forces. Loads are transferred from thestructure to the shear walls in-plane with the load. As a load istransferred into the wall, the wall will tend to move away from theload. The load therefore induces a rotation or moment in the wall. Wallrotation compresses one corner of the wall while the other corner tendsto lift off from the foundation or support. Traditionally, the lowercorners of the wall 8 have been held down by straps or hardware.However, as illustrated, the wall 8 may be retained by one or moretake-up mechanisms 10, according to the invention, to accommodatestructural shrinkage.

FIG. 1 illustrates two workable arrangements for securing an end of ananchor bolt 16. One arrangement simply bolts the sill plate 12 directlyto the foundation 14, perhaps also including one or more washers betweenthe retaining nut 18 and a sill plate 12. As illustrated, a take-up unit10 is also included to compensate for any wood shrinkage. Depending onthe loading of the take-up unit 10, a steel plate (not shown) may alsobe installed underneath the take-up unit 10 to spread out thecompressive force of the take-up unit 10 against the sill plate 12. Analternate arrangement is to secure one end of an anchor bolt 16incorporates a bracket 22. In this arrangement, the bracket 22 issecured to a stud 24 by multiple, spaced-apart fasteners 26. Again, atake-up unit 10 is included in position to compensate for woodshrinkage. A take-up unit 10 may be disposed between a retainer nut 18and a metal spacer platform base 28 of the illustrated typicalcommercial retainer arrangement. The wood stud 24 facilitates loadtransfer into the sill plate by distributing load into the sill plateover the entire stud end.

The illustrated take-up units 10 in FIG. 1 are installed around, andoriented to take up slack in an axial direction of, the anchor bolts 16.The take-up units 10 may be considered to be restrained from radialmotion by the anchor bolts 16. Preferred take-up units 10 are capable ofextending to a full height, and maintaining such a full height, even inthe event that the hold down system is subjected to excessive slack.

FIG. 1 illustrates a sill plate 12 installed directly on top of afoundation 14. This construction presents a minimum thickness of woodsubject to shrinkage in the hold down system. An alternate standardmethod of constructing a building is called platform framing. Thismethod includes building a floor platform on top of a double plated walland then adding a wall on top of the floor. Since the anchor bolt 16must then span a greater thickness of wood, a hold down to secure thewall on a floor of such construction is subject to considerable woodshrinkage. In another construction arrangement, a threaded rod 16 or ananchor bolt 16, attached to the foundation on one end, may pass througha sill plate 12, span the thickness of a joist, penetrate a subfloor andfloor, and the bottom member of a stud wall 8. The aforementioned woodenmembers are generally oriented to present a maximum amount of shrinkageto the anchor bolt 16. A combined shrinkage of about three-quarters ofan inch would not be atypical in such multilayer construction. Shrinkageas large as one-and-a-quarter inch may even be present.

A take-up unit 10 is illustrated in a fully collapsed, minimum installedheight, arrangement in FIG. 2. A bolt hole 32 receives a tie down bolt16 of a commercially available hold-down system. The bearing surface 34is typically configured to receive a retainer nut and washer, if desired(not shown). A recommended retainer nut includes a self lockingmechanism, such as a nylon collar. As an alternative, a thread lockingcompound may also be used between the anchor bolt 16 and a retaining nut18. A pair of retainer nuts may also be used as jam nuts in bindingopposition.

The illustrated sliding member 36 has a hollow shape, and carries aspring retaining fastener 38 and a deployment trigger 40. The slidingmember 36 may be formed integrally, or made and assembled as separatecomponents, such as separate cap and shell portions (not shown). Thesliding member 36 carries internal structure to interface in sliding andextending relation with the base member 42. A trigger 40 may befashioned as a pin or threaded fastener, or any other mechanism whichperforms as an adequate trigger. An exemplary trigger mechanism holds aunit in a pre-deployment, installation height until the unit isinstalled in a hold down system. Subsequent to such installation, thetrigger is disengaged to allow automatic height extension of a unit.Such disengagement is preferably simple and may be accomplished in thefield with a minimum of tools.

FIG. 3 illustrates a take-up unit 10 in an extended heightconfiguration. A maximum extension height is determined, in part, by thestrength of the material forming the sliding member 36, the base member42, and the cross section of the interface structure therebetween. Whenthe interface is fashioned as a thread, a sufficient amount of threadmaterial must remain in engagement having a sufficient cross-section tocarry the applied axial load. In addition, the load carried by a take-upunit may be applied eccentrically due to imperfections in the mountingstructure or alignment of the anchor bolt.

Note also, in FIG. 3, that a deployment trigger mechanism 40 has beenillustrated in an activated position. As illustrated, the trigger 40 isa threaded fastener. A fastener 40 may also serve as a motion limitingstop to prevent complete disassembly of a unit 10. To accomplish amotion limiting stop, a fastener 40 may be assembled in penetrationthrough a sliding member 36 in such a way as to restrict the range ofremoval of a fastener 40. A portion of the fastener 40 would then remainin engagement with a gap 44 machined in the threads 46 carried by a basemember 42. A sliding member 36 would be permitted to slide relative to abase member 42 only to the extent allowed by the fastener 40 incombination with a gap 44. Also visible in FIG. 3 is a socket 48, whichreceives the trigger mechanism 40 when a unit 10 is configured forinstallation height. A trigger 40, installed and seated in combinationwith a socket 48, prevents deployment of the unit 10 prior toinstallation in a hold down system.

The threads 46 are illustrated as being multi-start threads. Such amultiple-start thread configuration provides a larger change in heightof a unit 10 for a given rotation of a sliding member 36 relative to abase member 42 than does a single-start thread configuration. Currentlyit is desirable to provide the members 36 and 42 having between asingle-start thread and a four-start thread configuration. However, thenumber of thread starts may be increased to over eight, depending on therequirements of the application.

One trade-off to consider for a multi-start thread vs. a single-startthread is the tendency of a member 36 to slide backwards under load. Inthis context, “backwards” would be in the direction to decrease theheight of a unit 10. A multi-start thread has a lesser resistance tosliding backwards, compared to a single-start thread of equivalent size,because the friction force generated between meshing threads is lessenedby the increased contact angle possessed by a multi-start thread. Themulti-start thread has an increased lead length, or travel per rotation,which is equivalent to a steeper ramp. The “ramp” formed by a threadincreases in slope in a direct relationship with the number of threadstarts.

Of course, the friction force between the members 36 and 42 can beincreased by providing an interface surface having a higher coefficientof friction. One way to accomplish such an increase in friction would beto roughen the interface between mating thread surfaces. An alternativewould be to create interlocking teeth on the threads, or mutually wavythreads. Such interlocking teeth would provide a discontinuous increasein height of a take-up unit under load.

Another alternative might incorporate a simple spring loaded ratchetdevice in the member 36 and a series of vertical steps machined acrossthe member 42. In such an arrangement, the ratchet would engage thevertical steps, preventing “backward” movement. No Anti-Backing deviceor increased roughness surface treatment was needed in tests of theillustrated apparatus having a thread helix angle of about 5.5 degrees.The illustrated thread interface surface provides a smooth increase inheight of a unit, where such height increase may be described as acontinuous function of sliding displacement.

FIG. 4 is an exploded assembly view in perspective of a representativetake-up unit 10. A bias element, such as a coil spring 50, is receivedinterior to a bore 52. The spring 50 provides a practical,self-energizing source to slide a sliding member 36 relative to a basemember 42, thereby to extend a take-up unit 10 in height. Other biasingelements are within contemplation. Any other bias element capable ofperforming the desired function of urging a base member 42 and a slidingmember 36 in a direction to effect an increase in height of a unit 10would be acceptable.

The tab 54 of a spring 50 may be configured to serve as a retainingstructure to aid in assembly of a unit 10. During assembly, the spring50 is inserted into a member 36 where the tab 54 receives a fastener 38in retaining engagement. The tab 56 is then received by a slottedstructure within the base member 42 to secure the tab 56 relative to thebase member 42. The spring 50 may be shaped to be substantiallysymmetrical, providing equivalent structure at both ends. Such symmetrymay simplify manufacturing. A preload may be applied to the spring 50prior to engaging the sliding member 36 with the threads 46. The tabs 54and 56 may rotationally anchor the spring 50 to the sliding and basemembers 36 and 42 to provide torsional force acting to twist the slidingand base members 36 and 42 apart.

Still with reference to FIG. 4 and continuing the assembly procedure, amember 36 is then slid relative to a member 42 (by rotating one memberrelative to the other) until the trigger 40 may be engaged within thesocket 48. Following these assembly steps, the tie-down unit 10 isarmed, pre-loaded, and ready for installation in a wall hold-downfastening system.

After installing a unit 10 over an anchor bolt 16 (FIG. 1) and securingit with a retainer nut 18, the trigger 40 is released from engagementwith the socket 48. The unit 10 is then ready to extend in length andautomatically take up slack as the wood elements shrink. If the unit 10is accidentally activated without being secured by a retainer nut,engagement of the structure of the partially withdrawn trigger 40 withthe end of a slot 44 will prevent unintended disassembly of the unit.One reason to prevent such unwanted disassembly is to ensure that aproper preload will be maintained in the spring 50.

Preventing disassembly by engaging a safety mechanism after correctspring preload is established is a feature which may be included inpractice of the instant invention. FIG. 4 also illustrates how a slidingelement 36 may form a protective shield for the unexposed portion ofthreads carried by the base element 42, as well as the internal spring50. The unexposed portion of threads may be regarded as an advancinginterface. As the interface advances, an additional increase in take-upunit height is accomplished.

FIG. 5 illustrates an alternative apparatus according to the presentinvention. A portion of an alternative take-up unit 60 is illustrated ina cross-sectional view in elevation. The spring end tab 56 may beembodied as a straight pin end received in a slot structure embodied asa hole 62. Again, a spring 50 (only a portion of which is shown in FIG.5) may be symmetric for ease of manufacture. An anchor bolt slidinglypasses through the illustrated hole 64, formed in a base member 42, uponassembly of a unit 60 in a hold-down system. The threads 66 may includea section of trimmed threads 68 wherein the tips of the trimmed threadshave been removed to create a threaded section 68 having a reduceddiameter.

A portion indicated by the bracket 70 of the sliding member 36represents a sliding member 36 prior to assembly as a take-up unit 10.Prior to assembly, the end flange 72 protrudes at an angle and therebyclears all threads carried by the base member 42 during the assembly ofa unit 60. The portion indicated by the bracket 76 of a sliding member36 represents the configuration of a member 36 after the unit 60 isfully assembled and then substantially expanded in height. Note that theflange 72 has been deformed during the assembly procedure to behorizontal and in position to interfere with the thread tip 78.

During assembly of a unit 60, a sliding member 36 is threaded over abase member 42 until the flange 72 clears the threaded section 68. Aflange 72 is then “canned” or deformed to lie substantially in a planeperpendicular to an axis of a take-up unit 60. The flange 72 has areduced inner diameter subsequent to the canning operation. The reduceddiameter is such that an interference is created with untrimmed threadtips such as the thread tip 78. The interference created between theflange 72 and the thread tip 78 is another way to provide a safetymechanism to prevent inadvertent disassembly of a take-up unit. In thealternative, the flange 72 may be replaced by a separate snap ring (notshown) that can be interference fitted or otherwise attached to thesliding member 36 during assembly. The snap ring would then interferewith untrimmed thread tips in the same fashion as the flange 72.

FIG. 6 illustrates a take-up unit 60 in a perspective view from below.The illustrated unit has been activated to provide automatic heightadjustment, and is partially extended. The bottom bearing surface 88 hasa through hole 62 to receive a tab 56 from an internal spring 50 (seeFIG. 4). An alternate safety trigger mechanism is provided in theillustrated apparatus of FIG. 6. An oversize hole 90 slidingly receivesa fastener or actuation trigger (not shown) for engagement with thereceiving hole 92. In the alternative, the oversize hole 90 may havethreads to engage the actuation trigger, and the receiving hole 92 maybe made smooth to slidingly receive the trigger. With a fastenerinstalled through the hole 90 and secured in the hole 92, the unit 60 isin a configuration ready for installation in a wall hold down system.The fastener or actuation trigger is removed after such installation toactivate the automatic height adjusting capability of the unit 60. Inthis embodiment of a take-up unit 60, if the fastener or trigger wereaccidentally removed prior to installation, the flange 72 (seen in FIG.6 as the surface 94) would prevent undesired separation of the members36 and 42.

It is within contemplation for a flange 72 to have alternativeconfigurations which accomplish the same purpose as a safety mechanism.One alternative configuration might include discontinuous flangesections around the circumference of a member 36, rather than formingone uninterrupted circular section, as illustrated in FIG. 6. Anotherconfiguration might include an alternative flange as a section that maybe canned after final assembly to register into a discontinuous helicalgroove within a base member 42. Such a configuration combines aspects ofthe flange 72 of FIG. 5 and the trigger mechanism interface with thegroove 44 of FIGS. 3 and 4.

In situations where expected wood shrinkage or crushing might exceed thecapacity of a single take-up unit, two or more units may be stacked incombination, as illustrated in FIG. 7. Such an in-line configurationprovides an additive height extension capability. One circumstance wheresuch increased capability might be desired is in the construction of alog cabin. The situation might also arise in conventional constructionusing platform framing with joists having an extra depth, or inattaching an upper story wall to a “remote” foundation.

The embodiments of the take-up units 10 and 60 provide a measure ofprotection to the internal mechanism of the units. The illustratedsliding members 36 provide a cover over threads thereby protecting theinterface surface which may be used for further increase in unit height.Upon assembly with a retainer nut over a hole 32, the sliding member 36forms a substantial shield from debris and corrosive elements. Take-upunits are typically pre-lubricated with a dry lubricant prior toassembly further to promote smooth actuation over a long life. A lifespan of perhaps 30 years or more is appropriate for take-up units thatmay be enclosed within finished walls.

FIGS. 8A and 8B illustrate a third alternative embodiment of anapparatus in accordance with the invention, relying on translationalmovement between wedges, urged together using linear motion, instead ofrotary motion. These Figures depict a sectional view in elevation takenthrough a midplane of the apparatus, and include an anchor bolt 16 forperspective. FIG. 8A illustrates the apparatus in a pre-deployment,minimum installed height, configuration. A ramp member 102 serves as abase and receives in sliding contact a complementary sliding ramp member104. The slot 106 and the slot 108 are sized to receive an anchor bolt16 and allow the ramp members 102 and 104 to slide relative to eachother. In this embodiment, a washer 110 is typically included under aretaining nut 18.

Also illustrated in FIG. 8A is a deployment release trigger mechanismincluding a trigger 112 fashioned as a clevis pin. The release trigger112 is received by one end of a retainer pin 114. The pin 114 is securedto a sliding member 104 on its opposite end. The trigger 112 through thepin 114 maintains a ramp member 102 in proximity to a thrust base 116,thereby preventing premature height extension of take-up unit 100. It iswithin contemplation to replace the pin 114 and the trigger 112 with athreaded fastener passing through a thrust base 116 and threading intothe sliding member 104. An enlarged head section of such a fastener maybe sized to not pass through the thrust base 116. Such an alternatetrigger mechanism would simply be unscrewed from a sliding member 104 todeploy the take-up unit.

The thrust base 116 is illustrated as being structurally fixed to thebase member 102 by one or more fasteners 118. The thrust base 116 may besecured to the base member 102 by any other appropriate fasteningmethod, including without limitation, welding, interference fit, andadhesives. Furthermore, it is within contemplation also to machine anequivalent thrust base 116 directly from material forming the basemember 102.

A sliding member 104 and a base member 102 are typically joined in aslidable capture arrangement, which prevents separation of the membersin a height-increasing direction without a corresponding translationbetween base and sliding members. One arrangement to achieve such aresult is illustrated as the dovetail joint structure 120 forming adovetail joint between the members 102 and 104. Such a dovetail jointallows the members to slide relative each other in a height extendingfashion, but prevents vertical separation of the members. In the case ofan embodiment 100, a blind dovetail may be employed also to provide asafety mechanism to prevent a member 104 from sliding out of engagementwith a member 102 in the event of inadvertent trigger release.

Many other configurations to accomplish a slidable capture feature arewithin contemplation. For instance, illustrated components, including atrigger 112, may serve as the capture feature, as well as a deploymentrelease trigger mechanism. A thrust base 116, in combination with a pin114 and a trigger 112 may provide sufficient restraint from memberseparation prior to installation of a unit 100 in a wall hold downsystem. Threads between the sliding and base members 36 and 42 of thefirst embodiment 10 (FIG. 4) also serve as such a slidable captureinterface. Engaged threads prevent axial translation of separate memberswithout also producing a corresponding sliding motion.

With continued reference to FIG. 8A, it is preferred that a guidestructure of some sort be provided to prevent twisting of the basemember 102 and sliding member 104 relative to each other. Theillustrated dovetail joint structure 120 also provides such a guidingrestraint. A simple box joint also would serve as a sufficientrestraint. In a box joint configuration, the member 104 may bestructured as a cap, having a slot in which a member 102 may slide. Theconverse configuration is also workable, wherein a base member 102provides a slot in which a sliding member 104 may slide.

With reference to FIG. 8B, additional details, including theself-energized, height-extension capability of a take-up unit 100 may beseen. The arrangement illustrated in FIG. 8B represents a unit 100configured for a maximum installed height. An anchor bolt 16 preventsthe base and sliding members 102 and 104 from separating by slidingapart. The configurations of FIGS. 8A and 8B together demonstrate themaximum take-up height of which a single installed unit 100 is capable.Of course, two or more such units may be stacked end-to-end to achieve agreater take-up height.

Certain embodiments of a take-up unit 100 will have structure to preventbackwards movement of the sliding member 104 relative to the base member102. As with the rotationally actuated take-up units 10 and 60,“backwards” means motion of a sliding member 104 and a base member 102such that the overall height of the unit is reduced. In the case of acomplement ramp structure, as illustrated in the unit 100, the rampslope may be formed at such an angle that the frictional force generatedbetween the ramp members 102 and 104 is adequate to prevent suchundesired backwards travel. In certain situations, the interfacingsurfaces between members may be formed to have indexing teeth, similarto steps or ratcheting gear teeth. Alternatively, a spring loaded pawlmechanism may be carried by one member to interface in structuralinterference with teeth or other structure including serrations carriedby the other member.

In general, a unit 100 includes at least one, or a matched pair of,compression springs 124 to provide an automatic height extension forcewhen deployed in a wall hold down system. The springs 124 each arereceived within a socket in the sliding member 102 (not shown) and areloaded in compression during assembly of a unit 100. A locating dowel126 may aid in securing the free end of each spring 124 as the slidingmember is placed into an assembled, deployment configuration. Thepreload created by the springs 124 is countered by a trigger mechanism,including the trigger 112 in combination with a pin 114 and the thrustbase 116, prior to deployment of the unit.

Take-up units as illustrated and described may be manufactured from anysuitable material, including ferrous and nonferrous metals. At times,stainless steels may be preferred in certain applications, particularlyin corrosive or damp environments. Costs may be reduced in certaininstances by the use of mild carbon steels. The strength of a take-upunit is generally designed to exceed the strength of other components ofthe hold down system, such as the anchor bolt.

A take-up unit 10, 60, such as embodiments illustrated, is typicallyinstalled as a separate element, independent of threaded retainingelements of a hold down system. However it is within contemplation alsoto provide a standard, right-hand, threaded hole 32 (FIG. 2) tointerface with an anchor bolt 16. In such a configuration, a retainernut may serve as a jam nut, fixing the element 36 from rotation relativeto an anchor bolt 16. Such a configuration is presently considered lessdesirable because it removes one degree of rotational freedom from atake-up unit.

With a jam nut restraining the sliding member 36, only the base member42 need rotate to extend a unit in height. While still workable, such aconfiguration may be less reliable than simply allowing both the baseand sliding members 36 and 42 to rotate independently from the anchoringsystem. In the configuration having a threaded hole 32, it is oftendesirable to provide a left hand thread between the base and slidingmembers 36 and 42 to prevent rotating the sliding member 36 about ananchor bolt 16 under action of the self energizing spring 50. Anapparatus having similarly directed threads in both the hole 32 andbetween sliding members may potentially and undesirably unscrew itselffrom the anchor bolt.

The present invention may be embodied in other specific forms withoutdeparting from its structures, methods, or other essentialcharacteristics as broadly described herein and claimed hereinafter. Thedescribed embodiments are to be considered in all respects only asillustrative, and not restrictive. The scope of the invention is,therefore, indicated by the appended claims, rather than by theforegoing description. All changes which come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

1. An expandable washer comprising: a base of generally cylindricalshape; a slide of generally cylindrical shape threadedly engaging thebase to provide axial expansion of the washer during multiplerevolutions of the slide with respect to the base in a first directionand axial contraction of the washer during multiple revolutions of theslide with respect to the base in a second direction, opposite the firstdirection, the base and slide providing a clearance hole extendingaxially therethrough; and a coil spring having a first end engaging thebase and a second end engaging the slide, the coil spring containedcompletely within the base and slide and loaded in torsion to urgerelative rotation of the slide with respect to the base in the firstdirection.
 2. The washer of claim 1, wherein the coil spring is formedof stainless steel.
 3. The washer of claim 2, wherein the slidethreadedly engages the base with multi-start threads.
 4. The washer ofclaim 3, further comprising a stop defining a maximum axial expansion toresist separation of the slide from the base.
 5. The washer of claim 3,wherein the slide threadedly engages the base at a ramp angle of aboutfive and a half degrees.
 6. The washer of claim 5, wherein the stopimposes a structural interference between the slide and the base toresist further relative rotation once a selected axial expansion of thewasher is achieved.
 7. The washer of claim 6, wherein the stop comprisesa snap ring applied to the slide to structurally interfere with the baseonce a selected axial expansion of the washer is achieved.
 8. The washerof claim 6, further comprising a dry lubricant distributed along thethreaded engagement between the base and slide.
 9. The washer of claim8, further comprising a selectively removable trigger resisting rotationof the slide with respect to the base.
 10. The washer of claim 9,wherein the trigger comprises a removable screw extending radiallythrough the slide to engage the base.
 11. The washer of claim 1, whereinthe slide threadedly engages the base with multi-start threads.
 12. Thewasher of claim 1, wherein the slide threadedly engages the base at aramp angle of about five and a half degrees.
 13. The washer of claim 1,further comprising a stop imposing a structural interference between theslide and the base to resist further relative rotation once a selectedaxial expansion of the washer is achieved.
 14. The washer of claim 1,further comprising a dry lubricant distributed along the threadedengagement between the base and slide.
 15. The washer of claim 1,further comprising a trigger formed as a selectively removable screwextending radially through the slide to engage the base to resistrotation of the slide with respect to the base.